Whether flying helicopters or airplanes, sometimes it may be necessary to modify a flight plan enroute to incorporate an alternate route, altitude or destination. If plan A isn’t working, relax, there are plenty more letters in the alphabet.
Diligent planning prior to takeoff helps ensure that altering a plan in flight won’t be cause for concern. Whatever change is considered, it must be within the capabilities of the aircraft with regards to the amount of safe fuel onboard. The range of the aircraft, considering the amount of safe fuel and current environmental conditions, is called the radius of range (ROR). The ROR is never static, but constantly changing throughout flight.
Many of us fly aircraft over desolate areas of the West and Alaska, or offshore to oilrigs where refueling locations and helidecks are few and far between. Though helicopters have some advantages over airplanes, range and fuel endurance isn’t usually one of them. Beating the air into submission takes a lot of fuel, with a relatively high fuel consumption to weight ratio as compared to airplanes. On the other hand, while airplanes tend to have more range than helicopters, they are more restricted on where they can land. Both aircraft have their advantages, and all pilots need to continually assess available options which may be affected during flight due to changing weather and wind conditions, aircraft performance and condition, and the amount of safe fuel remaining. Safe fuel is the amount of fuel not including a reserve; the reserve being the greater of what is required by FAA regulation or what the pilot considers necessary.
Many pilots regard fuel planning as a linear calculation, where only the departure and destination points are considered. The formula for point of no return (PNR) is such a calculation that does not consider options off the route of flight. The PNR is a specific point along the route where should the aircraft fly beyond, it will lack sufficient fuel to turn back and safely land at the departure location with a reserve remaining. Below is the formula for PNR in minutes and conversion to PNR in miles.
Safe fuel (minutes) x GS2, divided by GS2 + GS1 = PNR (minutes)
PNR (minutes) x GS1, divided by 60 = PNR (miles)
- GS2 is the ground speed opposite of the course to be flown.
- GS1 is the ground speed on the course to be flown.
Using the two formulas one can determine the PNR for a flight in terms of the number of minutes flown and the number of miles flown. Subtract the PNR miles from the total route miles to determine the amount of miles remaining to the destination, which may be more useful when viewing a GPS or FMS. While PNR for a flight is a useful calculation, it is a linear 1-dimensional calculation and we can do much better adding more dimensions to our planning.
Most of the time a flight will have more options available than simply the departure and destination locations, and so we find the old PNR formula and that way of thinking to be insufficient. Let’s add another dimension and consider not just the route of flight, but also the possibility of changing course anytime should changing conditions dictate using the ROR concept.
I recently flew a helicopter from Anchorage to the Leonardo Helicopters factory in Philadelphia, which was more than 3200 nautical miles with 14 fuel stops. ROR flight planning was a critical aspect, especially during the first few days through Alaska and western Canada. However, it would increase the margin of safety for any cross-country flight, regardless of where one is flying.
The ROR is the distance the aircraft is capable of flying at any given point, and is represented by a large circle around the current aircraft position. The radius of that circle is dictated by the amount of safe fuel on board, cruise speed, and winds aloft. Let’s use a heavily loaded AW139 flying at 150 knots only carrying 1.3 hours of safe fuel as an example. In calm winds, the ROR at takeoff would be 195 miles, which is the maximum distance it could fly and still land with a reserve remaining. During flight as fuel is consumed the ROR will naturally decrease.
On this chart, the yellow circle depicts the ROR departing from Burns, Oregon. At this point, the aircraft is just starting to consume fuel and the ROR is at its largest. We can see at takeoff the destination is just within the ROR, indicating a planned landing with just the fuel reserve remaining. As the flight progresses and fuel is consumed the ROR decreases, and there becomes a point a little over halfway where returning to the departure point is no longer an option, corresponding to the PNR. The orange circle is the ROR at 93 nautical miles, the halfway point of the flight. As we near the destination the ROR continues to decrease to the point where many fewer options are available and at a certain point the only course of action is to land at the destination. The red circle is the ROR at 140 nautical miles. There are just four other airports within the ROR at 140 nautical miles and in another 20 nautical miles there won’t be any, other than the destination itself.
In the chart below we have essentially the same ROR chart, but with a 20-knot wind out of the west. As one can see, all the ROR circles offset downwind. This graphically shows that with a strong wind condition one is usually better off turning downwind for an alternate option, as there is more area within the ROR downwind than there is upwind.
Let’s add the third dimension to consider: altitude. Note that everything within the ROR may not necessarily be a viable option. It is possible that parts of the area within the ROR are further constrained by high terrain and weather. Maybe a ceiling prevents a climb in VFR to a necessary altitude in order to safely clear a mountain ridge east of course. Or maybe its getting near the end of the day when daylight will be waning and crossing a mountainous area in VFR flight without much illumination isn’t safe. We obviously don’t live in a flat world and must consider altitude.
The last dimension is time, and is considered throughout the flight. A prudent pilot will assess if weather currently reported is better or worse than forecast, and try to get an idea of what the trend is up ahead and near the destination. On this particular flight a pilot would make early and careful assessments as to winds aloft, changing current weather conditions, and amended forecasts along the route of flight and areas inside the ROR. Should one encounter a worse than planned condition, such as a stronger headwind or worse than forecast weather enroute, making a decision to alter the planned flight in the early stages is better than in the later stages. In the early stages more options are available and one can carefully choose the best, whereas in the later stages of the flight options will have dwindled along with fuel. With deteriorating conditions this flight could evolve into a situation supporting the old adage; a superior pilot uses superior judgment to avoid the necessity of using superior skill. As we know, superior judgment for a pilot is using all available information to determine what risk may present in the future, and then determine a course of action to avoid or mitigate that risk.
The final chart is an example how a pilot might incorporate terrain issues, forecast weather and NOTAMs into the ROR chart. One can add notes along with the depicted ROR circles. Maybe weather forecast to the west towards Pendleton and Walla Walla indicates marginal VFR conditions. One may consider taking those areas out of the ROR, especially coming from higher terrain where it may not be possible to safely get under a cloud layer. To the east is higher terrain, which may block access in that direction; it is also the windward side of a mountain area, which tends to collect a lot of cloud cover. Also noted is a NOTAM for an airport along the route of flight for fuel out of service, though it could still be used for a safe landing as one of the last options. Airports circled in green highlighter represent good enroute options should a diversion with landing become prudent, such as when facing deteriorating weather or an aircraft problem of some kind.
Of course if things really get bad, let’s call it plan Z, we can usually find a place to just land. Helicopters definitely have an advantage over airplanes for landing off-airport, though I’ve seen some amazing bush pilots in Alaska. Plan Z is certainly better than running out of fuel or flying into dangerous weather, and sometimes, JUST LAND is the best option. During my power line patrol days in the 1980s, I knew many of the farmers along the route from visits during the summer. These farms made for some good alternates during the winter, when I would occasionally land for a welcomed cup of coffee to wait out the odd snowstorm.
The ROR certainly doesn’t provide everything a pilot needs to think about but it does help with a graphic visualization of areas available throughout the flight. Next cross-country flight, get a sectional chart out and make some ROR circles using a highlighter along the route of flight. Use any color and at any increments you desire. Remember to offset the ROR circles downwind, in relation to wind speed and time of flight. For example, a 40-knot wind would have an offset of 40 nautical miles for an hour flight, 60 nautical miles offset for a 1.5-hour flight, and an 80 nautical mile offset for a 2-hour flight.
Once in flight, there isn’t much a pilot can do to alter the aircraft ROR. Consider how slowing down to a maximum range cruise speed will increase the ROR. Hopefully, your Rotorcraft Flight Manual will have fuel consumption charts, if not you will have to rely on past experience for fuel consumption rates.
Maybe someday flight planning and moving map apps, such as Foreflight will provide an enhanced ROR map overlay option, but for now a couple of colored pens and a trusty sectional chart will suffice.